Platform for recovering solar energy

ABSTRACT

In a device designed as a rotatably mounted platform (a) for recovering solar electricity, the focusing roof layer (3) further deflects the incident radiation so that the light beams (122, 123, 126) formed by the concentrating optical system are incident approximately perpendicularly on the radiation converter (112) arranged underneath.

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 07/898,160 filed Jun. 15, 1992, U.S. Pat. No. 5,286,305.

FIELD OF THE INVENTION

The invention describes a platform which is supported by a water layer,experiences a daily rotation about the vertical axis at the angularvelocity of the sun and is rotated back during the night. To follow thesolar altitude, the incident radiation is refracted towards the verticalto such an extent that all refracted sunrays strike the radiationconverter.

PRIOR ART

Solar energy converters which are arranged on a body of water and trackthe azimuth are known. Their disadvantage is that the concentrators haveto be oriented according to the particular solar altitude, which entailsconsiderable mechanical effort. A second disadvantage is that thepivotable concentrators give rise to high drags so that wind forcescause the total installation to vibrate. In addition, all largecomponents exposed to wind forces must be constructed using anappropriate quantity of material. A third disadvantage is thatconcentrators placed one behind the other viewed in the direction of thesun must be installed such a large distance apart that they do not castshadows on one another.

SUMMARY OF THE INVENTION

The object of the invention is the realization of two axis tracking inthe case of a platform whose surface presents no end surfaces to thewind and in which furthermore the entire area of the platform is used asan aperture area owing to the absence of mutual shading.

In order to realize this, the invention envisages a platform having ametallic base which has parallel channels whose lower surfaces extendclose to the base of a water tank and which carry the photocells, sothat the heat loss is passed through the base into the water and inparticular into the water volume located at a higher level. The sun'srays are guided over as large a solar elevation interval as possible bya cylindrical lens system which preferably consists of several lensarrangements located one underneath the other. The invention givespreference to a system in which triangular prismatic channels whosenon-transmitting boundary faces are mirrored are formed between the lensarrangements. These lens arrangements substituting mechanical trackingof the solar elevation and referred to below as tracking lenses arearranged above a concentrating Fresnel lens. An advantageous embodimentenvisages the integration of a tracking lens with a Fresnel lens. Theinvention furthermore relates to means which ensure that the photocellsare irradiated as uniformly as possible. As a first measure, thephotocells float in troughs and follow the height movement of a focalline. In a more advantageous solution, secondary lenses are used forcompensation of the shift in height. It proves to be advantageous if theradiation is guided in such a way that the outward-facing layer of thehorizontally oriented lens arrangement has an upward-facing first,smooth boundary face and possesses, on the downward-facing side, a groupof secondary boundary faces divided into steps, the boundary faces ofthe steps making an angle with the first boundary face such that,optionally in cooperation with further lens arrangements, a sunray whichmakes an angle of more than 60° with the vertical is refracted to givean emerging ray which makes an angle of more than 110° with thedirection of incidence of the sunray and at the same time and,simultaneously with the vertical, makes an angle of less than 30° with alimb opposite the direction of incidence, while a sunray whose angle ofincidence relative to the vertical is less than 20° is refracted to givean emerging ray which points towards the sun and whose limb pointstowards the sun and which makes an angle of less than 30° with thevertical, so that the emerging rays are concentrated onto a radiationconverter. In the most advantageous solution, the angles of incidencehave equal magnitudes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a platform having an edge zone.

FIG. 2 shows a cut-out II from FIG. 1, in vertical section.

FIG. 3 shows a perspective view of a lens arrangement.

FIG. 4 shows the beam path in a refined version.

FIGS. 5a and 5b show a flat lens system with facets.

FIG. 6 shows a flat lens with primary and secondary steps.

FIG. 7 schematically shows the paths of curved steps.

FIG. 8a shows the beam path in a secondary lens cut vertically and atright angles to the direction of the sun.

FIG. 8b shows the beam path in the same secondary lens with lateralshifting of a light beam at right angles.

FIG. 8c shows the beam path in the same secondary lens with lateralshifting of an oblique light beam.

FIG. 9 shows an embodiment whose cross-section is composed of trapezoidslying one on top of the other.

FIG. 10 shows a secondary lens which consists of a lens which convergesin the outer regions and diverges in the inner region and is supportedby mirrored walls.

FIG. 11 schematically shows a cross-section through channels in whichphotocells float.

FIG. 12 shows a secondary lens for compensation of lateral shifting ofthe light beam by displacement of a component.

FIGS. 13a, b, c show a comparison of the deflections for different solarheights.

DETAILED DESCRIPTION

FIG. 1 schematically shows the solar power station according to theinvention, having a platform a, rotatable about the vertical axis d, anda frame b which floats in a channel c.

FIG. 2 shows a horizontal section of the cut-out II in FIG. 1. Theenergy conversion is effected on a circular platform a which issupported by a thin water layer 2 It is divided into, parallelconcentrator tunnels 5. A toroidal pipe 6 surrounding the platform formsa frame for a network which keeps the concentrator tunnels an exactdistance apart and which transmits the rotary movement imposed on thetoroidal pipe 6 to the platform. The network is formed from sheet metalsections 7 and steel cables 8 and is kept a constant distance from metalbase sheets 1 by means of thin-walled flat tubes 9. A channel 10 inwhich the photocells 4 are located runs along the central line of eachconcentrator tunnel 5. The cover consists of concentrator discs 3 whichnot only concentrate the sun's rays in the manner of a cylindricalFresnel lens onto focal lines but also refract the resulting light beamsdownwards. The photocells 4 are located between strand-like secondarylenses 12 and the bottom of the profile channel 10. The platform iscentered by the toroidal pipe 6 which rolls along the annular wall 21 oncasters 13, and is rotated about the vertical axis d at the angularvelocity of the sun, so that during the day the photocells, controlledby known means, always follow the azimuth of the sun. During the night,the platform is rotated back to the starting position. The secondarylens 12 guides the light beam, independently of the level of theparticular focal line, to the photocell 4 which covers the underneath ofthe secondary lens. A light beam 15 striking vertically is shown in theconcentrator tunnel 5. The gap between the toroidal pipe 6 and the wall16 is bridged by a contacting film 17. In addition, a film 18 is clampedbetween the toroidal pipe 6 and the adjacent energy tunnel so thatevaporation of water is prevented. The heat dissipation of thephotocells 4, unless already transferred during the hours of sunlight,is passed into the water layer 2 which is separated by a film from theground 19. The energy stored by the water is emitted 24 hours a day byconvection and infrared radiation. The electricity generated by theplatform is passed via flexible conductors into an earth cable leadingto the center. Rainwater passes through the flat tubes 9 into the waterlayer 2, from which excess water can flow away.

FIG. 3 shows a perspective view of a lens arrangement which consists ofan upper layer 30a having steps 30e and a layer 30b underneath havingsteps 32a and 32b, which enclose channels 31 between them. A Fresnellens 33 whose steps 34 are at right angles to the steps 32 is locatedbelow this stepped lens 30a, 30b. The layers 30a and 30b have verticalchannels 37 into which sheet metal strips 38 are inserted so that thelayers cannot move towards one another. The flanks 39a and 39b arepreferably mirrored. For regions with sand storms several layers 35 ofan extremely thin film are fastened on the outside of the lensarrangement. If the surface of the uppermost layer 36 in each case isscratched, the said layer is removed. It has proved advantageous to bendthe edge 30c downwards so that the line 30d is curved, resulting incurved roof elements according to FIG. 2. This applies correspondinglyfor all flat lens variants.

FIG. 4 shows a cross-section through a lens arrangement 30 with the raysof the sun at 20° and 80° elevation. While the rays 40a emerge as ray40b after passing four times through boundary faces, said rays making anangle of 18° with the vertical 54, the ray 41a must be reflected at theflank 45b. For this reason, the flanks 45a and 45b are mirrored. Afterpassing twice through boundary faces, the ray 42a passes as ray 42cthrough the channel 46 and emerges as ray 42b at the same angle to thevertical as the ray 40b but with opposite sign. After double refraction,the morning ray 41a with a very obtuse angle of incidence enters thechannel 46 as ray and is then reflected to become ray 41d. This ray 41dis parallel to the ray 42c, so that 41b, too, emerges at an angle of+18° to the vertical 54, as in the case of ray 42b. All sunrays strikingbetween the limits of the morning incidence of 20° and the middayincidence of 80° lead to emerging beams which lie within the interval40b-41b of the angles of emergence. After triple refraction, the ray 43astrikes the flank 45b, undergoes total reflection there and becomes ray43e. It then emerges as ray 43f and is reflected at the mirrored flankregion 48a so that it emerges as ray 43b within the allowed angularinterval. All 80° rays between the rays 44a₁ and 44a₂ would emergeoutside the allowed angular interval if the flanks 45a and 45b were tomerge with one another. To prevent loss of the rays, the channel 46having the faces 46a and 46b through which rays pass is bordered by achannel 47 which has the faces 47a and 47b through which rays pass andwhose walls through which rays pass guide the rays 44a₁ and 44a₂,optionally in combination with a reflection at the upper mirrored strip48a of the flank 48a, 48b, in such a way that they emerge in the allowedinterval. The rays between the rays 45a₃ and 45a₄ undergo totalreflection at the flank 48b and then emerge within the allowed angularinterval. The ray 44a₅ emerges as ray 44b with a negative angle, and thesubsequent rays up to 44a₆ undergo total reflection at the flank 45b andemerge with a positive angle in the allowed angular range, as does theray 43b. All rays to the right thereof pass through the cylindrical lenswith a positive angle, as in the case of the ray 42a.

FIG. 5a schematically shows an embodiment of the lens arrangementaccording to FIGS. 3 and 4, in which the downward-pointing steps 32 arecombined with the perpendicular steps 34 of the Fresnel lens underneathto give faceted lenses. The flat upper surface 50 pointing upwards formsthe incident surface. In the planes at right angles to the incidentsunlight 53, the rectangular regions 51 make the angles x₁, x₂, x₃, . .. with the vertical 54, which angles become 90° in the region of thesymmetry line 52. However, the rectangular regions 51 pointing towardsthe photocell are simultaneously inclined at the constant angle y to thevertical 54, in planes containing the sunrays 53. This gives rise tolight beams which are simultaneously refracted towards the vertical 54.The planes of the flanks 55a, 55b, etc. intersect one another close tothe focal line, in order to prevent shading of the emerging radiation ofthe relevant subsequent step. The flanks 56 at right angles to thesymmetry line 52 are all in planes which make the constant angle waccording to FIG. 4 with the vertical 54.

In FIG. 5b, the rays 57a and 57b form the peripheral rays of the lightbeam produced at the elevation angle u of the sunrays 58. The peripheralrays 59a and 59b, which are assigned an elevation angle of 60°, are in avertical plane.

FIG. 6a shows another embodiment of a lens according to the inventionwhich has steps 60 at right angles to the plane of incidence, so thatsunrays 61 having a very obtuse angle of incidence are refracted towardsthe vertical 54. The rays having a very acute angle of incidence arerefracted towards the sun side. The surfaces of the steps 60 are in turnprovided with steps 63 of substantially finer division.

FIG. 6b shows the steps in the framed region VI on a larger scale. Thesteps 64, 65, 66 have prism angles which point towards the symmetry line67 and become zero at the height of the symmetry line. These stepseffect the concentration.

FIG. 7 shows a flat lens viewed from below. Steps 71 whose prism anglesassume zero value at the height of the vertex line 70 and increase onboth sides with increasing distance from the vertex line 70 runsymmetrically to the vertex line 70 along virtually parabolic lines.Each viewing element of the steps 71 which is a distance away from thevertex line 70 has a wedge angle which, in conjunction with the anglewhich the tangent to the viewing element makes with the vertex line 70,refracts a sunray passing through from the (invisible) upper sideoptically skew to a focal line.

FIG. 8a shows the cross-section of a cylindrical lens 80 which is inoptical contact with the photocell 81. This contact is achieved byintroducing a wetting immersion liquid or an optical cement 82 having atailored refractive index. The geometry meets the requirement that thesolar cell 81 should be illuminated with virtually identical luminousintensity for all existing light beams having the peripheral rays 87aand 87b. This is achieved by a cylindrical lens whose cross-sectionalshape is trapezoidal in the lower part and corresponds to an asphericalplano-convex cylindrical lens in the upper part. In a preferredembodiment, the incident surface 83 has an elliptical sectional curve.The lateral surfaces 84 are likewise in the form of optical functionalsurfaces, so that total reflection takes place where required. The focallines 85a, which would be located below the photocell 81, are producedabove the photocells 81 owing to the shape of the incident surface, sothat the focal line does not migrate through the cell at any solarelevation. The migration of the focal line would assume the magnitude85b without a secondary lens. By means of the secondary lens, the focalline is positioned in such a way that, when the photocell 81 is suitablycoordinated with the incident surface 83, an essentially uniformdistribution of the luminous intensity over the cell width is achieved,corresponding to blurred focusing of the focal lines onto thephotocells.

Where there is a lateral shift between the subsequent lens and thesecondary lens owing to interfering forces when the power station isbeing operated, the incident surface 83 effects an optical compensationof the shifting of the focal line. While in the case of perpendicularlight beams having the peripheral rays 87a the outer rays are caused toconverge, the rays of an oblique light beam having the peripheral rays87b experience divergent refraction.

FIG. 8b shows the refracted rays 88a of the vertically incident lightbeam having the peripheral rays 87a, the secondary lens 80 being shiftedlaterally relative to the light beam by a magnitude 89.

FIG. 8c shows the beam path for the oblique position of the light beams122 and 126 having the peripheral rays 87c in the case of a lateralshifting by the magnitude 89, said oblique position being shown in FIGS.13a and 13c.

FIG. 9 shows a further embodiment of a secondary lens which isassociated with the photocell 94 and whose effect corresponds to that ofthe secondary lens of FIG. 8. Considerable material is saved accordingto the smaller cross-sectional area, and less attenuation is achieved asa result of the shorter light paths for the peripheral rays. Theembodiment of the incident surface 83, described in FIG. 8, is dividedinto the areas 90, 91, 92 and 93. Here too, all lateral surfaces aredesigned so that total reflection of the rays which may be incident frominside is ensured.

FIG. 10 shows a secondary lens whose outer regions 103a and 103b effectconvergent refraction of the outer regions of a light beam which is in avertical plane, whereas they effect divergent refraction, through themiddle region 104, of these rays of a light beam in an oblique plane.The lateral walls 101 are mirrored, with the result that evenhorizontally shifted rays are reflected to the photocell 102. If thelens region is produced from organic glass, the space 105 underneath,including walls 101, can be produced integrally with the lens region.

FIG. 11 shows a cross-section of a concentrator tunnel 115 having achannel 110 in which a pontoon 111 floats on a body 116 of water andholds the photocell 112. The height of the body 116 of water in thechannel determines the distance of the photocell 112 from the flat lens114.

FIG. 12 shows a secondary lens 116 which is arranged so that it can bedisplaced laterally relative to the photocell 112a by the magnitude 117aand which effects the shift by a maximum magnitude 117a by means of anapparatus which is not shown, as a function of the lateral shift of thelight beam relative to the photocell 112a. As a result, the light beamhaving the peripheral rays 118a is guided towards the photocell 112aeven when these peripheral rays have shifted up to the distance 117 fromthe symmetry line 117 into the position 118b.

FIG. 13a shows a light beam which is produced by the flat lensesdescribed at the outset. The greatest refraction about axes 120 at rightangles to the focal line 121 takes place in the early morning and lateafternoon. The refracted light beam 122 makes an angle of about -18°with the vertical. In this oblique position of the light beam, the focalline 121 has the greatest height.

FIG. 13b shows that the light beam 123 is perpendicular twice a day, ineach case when the solar elevation is about 60°. The theoretical focalline 124 is then below the photocell 112 which collects the totalconcentrated radiation. It is even below the channel 110 and thusreaches the lowest level.

As shown in FIG. 13c, the noon sunrays 125 are refracted about the axis120 at right angles to the focal line towards the sun, so that the lightbeams 126 lie in planes which make an angle of +18° with the vertical.The focal line 127 once again reaches the same height as the focal line121 does in the early morning and late afternoon.

We claim:
 1. Platform for solar power stations rotatable about avertical axis (D) having channels (10, 110) floating on a liquid layer(2), and a transparent roof (3) above the channels (10, 110) whichrefracts sunbeams (122, 123, 126) downwards and concentrates theincident radiation onto photocells (4) arranged below the roof (3),characterized in that the photocells (4) are in heat-conductingconnection to the channels (10, 110), and that the channels (10, 110)are formed in such a way that the heat dissipation of the photocells (4)is predominantly passed into the liquid layer (2) through a region ofthe channels (10, 110) which lies below the surface of the liquid layer(2).
 2. Platform according to claim 1, characterized in that theoutward-facing layer of the transparent roof (3) forms a horizontal flatlens (30a) having a first outward-facing smooth boundary face (50) whichhas, on its downward-facing side, a group of second boundary faces whichare divided into steps (30e) and, in the operating position, runtransversely to the sunrays, the steps (30e) making an angle with thefirst boundary face (50) such that, optionally in coordination with asecond flat lens (36b) having steps (32a and 32b) and optionally with aflat lens (33) having steps (34) perpendicular to the stated steps, asunray. (40a) having a zenith angle of more than 60° is refracted togive an emerging ray (40b) which makes an angle of less than 30° withthe vertical in a direction away from the sun while a sunray (42a)having a zenith angle of less than 20° is refracted to give an emergingray (42b) which points towards the sun and makes an angle of less than30° with the vertical (54), and that the rays emerging from thetransparent roof (3) are concentrated to a focal line (121, 124, 127)parallel to the sunrays.
 3. Platform according to claim 2, characterizedin that the rays (40a and 42a) bounding an interval of zenith angles arerefracted to give their associated emerging rays (40b and 42b), each ofwhich make with the vertical (54) an angle of virtually the samemagnitude but opposite sign.
 4. Platform according to claim 2,characterized in that the downward-facing steps (30e) of theoutward-facing flat lens (30a), together with upward-facing steps (32a)having the same step division of a flat lens (30b) located underneath,enclose prismatic channels (31).
 5. Platform according to claim 4,characterized in that grooves (37), into which thin-walled, tape-likestrips (38) which fix the flat lenses (30a and 30h) to one another inthe direction of the channels (31) project, run at right angles to thechannels (31).
 6. Platform according to claim 4, characterized in thatthe boundary faces (46b, 47b) of each step of the outward-facing flatlens (30a), through which boundary faces rays pass, and the boundaryfaces (46a, 47a) of each upward-facing step of the flat lens (30b)underneath, through which boundary faces rays pass, enclose twotriangular channels (46, 47), the larger (46) of which tapers towardsthe sun in the operating position and the smaller (47) of which widenstowards the sun in the operating position.
 7. Platform according toclaim 2, characterized in that the boundary faces of the steps 30e and32a) of the flat lenses (30a and 30b) lying one of top of the other,through which boundary faces rays pass, consist of parallel-strip-likeregions (46a and 47a, 46b and 47b) which are present side-by-side andmake an obtuse angle with one another.
 8. Platform according to claim 2,characterized in that the flanks of the downward-facing steps of thesecond flat lens (30b) have strip-like regions (48a, 48b) which lie oneon top of the other and make obtuse angles with one another.
 9. Platformaccording to claim 2, characterized in that the downward-facing side ofthe transparent roof consists of facets (51) having edges parallel andat right angles to the perpendicular plane of incidence, those surfacesof the facets (51) through which rays pass being inclined relative tothe horizontal so that a vertical section through the facets (51), atright angles to the vertical plane of incidence, is equivalent to asection through a Fresnel lens, and that a section through the facetsparallel to the plane of incidence exhibits a periodic step. 10.Platform according to claim 2, characterized in that flank-formingsurfaces (45a, 45b, 48a, 48b) of roof-forming flat lenses through whichrays entering the interval (40h, 41b) of angles of emergence do not passin the operating position are mirrored, and that these mirrored surfaces(45a, 45b, 48a, 48b) make an angle with the vertical (154) such thatrays (41c) incident on these flanks are reflected in a direction suchthat they emerge within the interval of angles of emergence which isformed by only refracted sunrays (40a and 42b).
 11. Platform accordingto claim 4, characterized in that a first group of downward-facing steps(60) of the roof-forming flat lens has a coarse division, and that theboundary faces of these steps (60), through which boundary faces rayspass, carry secondary steps (63) which are at right angles to the steps(60) and have a finer division with the varying angles of a Fresnellens.
 12. Platform according to claim 1, characterized in that thetransparent roof (3) contains a flat lens with prism-forming steps (71)which run along curved paths symmetrically to a vertex line (70)parallel to the plane of incidence, the prism angles increasing over thelength of the steps with increasing distance from the vertex line (70),and furthermore that the prism angles associated with each distance fromthe vertex line (70) and the angles between the respective tangents tothe step (71) and the vertex line (70) are chosen so that all incidentsunrays (61) are refracted towards a focal line.
 13. Platform accordingto claim 1, characterized in that a secondary lens (12, 80, 103, 104)whose optical geometry guides light beams, which, as a function of therespective zenith angle, generate focal lines (121, 124, 127) havingdifferent distances from the transparent roof (3), within the limits ofthe predetermined interval of zenith angles, onto the photocells (4, 81,94) is arranged between the transparent roof (3) and the photocells (4,81, 94).
 14. Structure according to claim 13, characterized in that thephotocells (4, 81, 94, 112a) are connected to the secondary lens (12,80) without any optical distance in between.
 15. Platform according toclaim 13, characterized in that the geometry of the secondary lens (12,80, 103, 104) guides the sunrays refracted to give light beams (122,123, 126), provided that said sunrays are laterally displaced relativeto the photocells (81, 94, 102) when they are incident on the secondarylens (12, 80, 103, 104,) in such a way that these light beams (122, 123,126) are all incident on the photocells (81, 94, 102) within apredetermined interval (89) of the lateral shift.
 16. Platform accordingto claim 13, characterized in that the shape of the entry surface (83,83a) of the secondary lens 80 is such that the focal lines are withinthe secondary lens (80) so that they do not move through the photocells(81) at any zenith angle.
 17. Platform according to any of claim 13,characterized in that the cross-section of the secondary lens (80) formsa trapezium whose lower side faces the photocells (81, 94, 102) andwhich has symmetrically divergent sides (84) and an upper side (83)which is formed from three parts and consists of parts (83a), whichresemble sections of a longitudinally cut upper half of an ellipsehaving a horizontal long axis, and a middle part (88b, 90, 104) whicheffects divergent refraction of obliquely incident rays (87b). 18.Platform according to claim 13, characterized in that the lower side ofthe secondary lens (80) faces the photocells (81, 94, 102) and that thecross-section of the secondary lens has wall regions which aresymmetrical to the vertical and are formed from sides of trapezoidswhich are present one on top of the other, and that the upper side (90)resembles an arc of a circle.
 19. Platform according to claim 13,characterized in that a cylindrical lens (104) whose outer regions(103a, 103b) effect convergent refraction are kept a predetermineddistance above the photocells (102) by mirrored wall regions (101). 20.Platform according to claim 1, characterized in that the photocells(112) themselves are supported, in such a way that they float, by a body(116) of liquid, by means of whose level the distance of the photocells(112) from the transparent roof (114) can be adjusted.
 21. Platformaccording to claim 1, characterized in that an element of a module whichconsists of a photocell (112a) having a central line (119) and asecondary lens (116) or a part of such a secondary lens is arrangeddisplaceably relative to another such element, the distance (117a)between the central line (119) and the symmetry line of the secondarylens (116) being variable, and that, as a function of the shifting ofthe zone of highest luminous intensity from the photocell (112a), meanseffect a relative displacement which brings the photocell (112a) and thezone of highest luminous intensity into coincidence again.
 22. Platformaccording to claim 1, characterized in that several layers (35) of anextremely thin removable film are arranged on the transparent roof (3,30, 50).